In electromagnetic or sound transmission systems, an array transducer uses an array of simple transducers. Each transducer forming the array is known as an element of the array. The signals emitted from the elements are linearly combined as a weighted sum to form a receive array. A common signal is fed to the elements and weighted to form a transmit array. The process of combining the signals emitted from or fed to different elements is known as beam forming. When signals are combined without any gain and phase change, the direction in which the array has maximum response is perpendicular to the line joining all elements of a straight linear array.
The signals from or the common signal fed to each element of the array may be weighted in amplitude and phase. Phasing, or time delaying, points the maximum response of the array, or the beam center, in a desired direction. The amplitude weights affect the total signal gain of the array, the width of the main beam, and the level of the array response in directions outside the main beam (that is, side lobe levels). For frequency independent amplitude weights and for phasing which time delays all elements to add up in phase in response to an incident plane wave in a particular direction, the beamwidth of an array transducer decreases with increasing frequency. For wide area search purposes, it is often desirable to transmit acoustic power over a large angular sector with uniform coverage over the sector independent of the frequency. Thus various frequency dependent element weighting methods have been sought to counteract the natural tendency of the array beams to get narrower with increasing frequency.
One such method is to invoke linear superposition and simultaneously form a fan of contiguous narrow beams that together cover the desired broad angular sector. The number of beams must increase and the angular interval between steering directions of adjacent beams must decrease with increasing frequency in order to maintain relatively uniform sector coverage. When the implications of this approach on element amplitude weights are analyzed, it is found that the element amplitude weights are reduced appreciably and vary in sign outside of a central core region on the array and that this central core region decreases in extent with increasing frequency. Thus, simultaneous steering of the full array to many overlapped steering directions, in order to maintain frequency-independent angular sector coverage, is equivalent to shortening the effective aperture of the array with increasing frequency. Since not all the available elements are being driven to maximum or even appreciable amplitude, the power output of the array is reduced and this reduction becomes more severe with increasing frequency.
Accordingly, a need in the art exists for systems and methods for weighting the elements of an array transducer in order to maintain constant beamwidth over a large frequency range while transmitting maximum power.
Accordingly, the invention provides a system and method for transmitting maximum power over large angular sectors independent of frequency. The invention also provides a system and method for receiving incoming plane wave signals with constant signal gain over large angular sectors independent of frequency.